Abrasive Flow Machining (AFM) has gained wide acceptance for a number of applications as the machining and finishing technique of choice. Such techniques are particularly well adapted, for example, to working interior passages in work pieces, for light grinding, deburring, radiussing, leveling, and polishing of complex surfaces, and particularly three-dimensioned surfaces where surface detail requires working, and in repetitive working of multiple work pieces of complex form and shape.
In certain applications, abrasive flow machining involves passing a viscoelastic medium containing an abrasive through orifices that require substantial uniformity through or across the surfaces to be worked. The viscoelastic medium functioning as a carrier for the abrasive also provides the working force to the abrasive as the abrasive is forced through an orifice or carried across the surface. The medium flows to conform to the opening or surface of the work piece. Thus, one of the advantages of the process is to fill passages and span work surfaces when placed between a work piece surface and a member designed to confine the flow and constrain the medium in engagement with the surface of the work piece.
In many contexts, particular advantages are attained when the viscoelastic abrasive medium is also rheopectic, i.e., increasing in apparent viscosity with applied stress. With the appropriate application of stress, typically either shear or compressive stress, to the medium, it is possible to substantially attain plug flow of the medium across the surfaces of the work piece to be worked in the operation. Substantially higher working force is applied to the surface by such plug flow when compared to viscous flow of the medium. A description of the basic prior art on abrasive flow machining can be found in U.S. Pat. Nos. 3,521,412, 3,634,973, McCarty and 3,819,343, 5,125,191, Rhoades.
In the use of abrasive flow machining to provide the simultaneous precision machining of parts which are initially manufactured with variations outside of desired tolerances, it is difficult to regulate the flow of media through the parts, because the process is based upon fluid or plastic flow of the medium through a restricted opening or passage in the work piece such as an opening or orifice. Examples of these workpieces include turbine blades, cast intake manifolds, extrusion dies, diesel injector nozzles, and the like. Since the work being done is internal, it is difficult to know when the processing has been completed to exact specifications. In addition, the abrasive flow machining of these complex internal passages is often complicated by the number of dynamic variables involved in the process, including the condition of the abrasive medium, grit sizes, temperatures, extrusion pressures and the volume of media extruded. Because of the interrelationship of the various factors, it is often impossible to determine when a part in process has attained the desires performance criteria. In such cases it is necessary to profile a known functional fluid flow through the parts to determine by repeated tests whether or not the processing has been completed. This uncertainty can lead to the expenditure of unnecessary effort; either in needing to re-process a component to achieve the desired specifications or in the over-processing of components beyond what is required.
It has been shown that there is a correlation between the root mean square (RMS) of the voltage signal of acoustic emission and non-abrasive flow machining conditions found in abrasive machining processes such as grinding or mechanical abrasive deburring, see, e.g., Dornfeld, D. A, and H.G.Cai, 1984, "An investigation of Grinding and Wheel Loading Using Acoustic Emission," ASME Journal of engineering for Industry, Vol. 106. pp.28-33; Dornfeld, D. A. and Erickson, E., 1989, "Robotic Deburring With Real Time Acoustic Feedback Control," Mechanics of Deburring and Surface Finishing Processes, PED, Vol. 38, Eds. Stango, R. J. and Fitzpatrick, P. R., ASME, New York, pp. 13-26; Dunegan, H. L. 1999, "Modal Analysis of Acoustic Emission Signals," Journal of Acoustic Emissions, Vol. 15, 1-4.
In addition to RMS investigations, other traditional acoustic emission characteristics such as zero crossing rate, rise time, pulse width and Kurtosis have been studied. The analysis of traditional acoustic emission techniques to abrasive flow machining has been described. It is known that the amount of material removed from a part is related to an improvement in surface finish. A correlation has been sought between the acoustic energy (RMS.sup.2 of the signal) and the depth of cut and, thus, a correlation with enhancements in the surface finish. The relationship, RMS=(C*Ac*V).sup.1/2, where C is a constant depending upon machining conditions and material, V is the media velocity and Ac is the cross sectional area of an extrusion passage, is sensitive to extrusion pressure and other abrasive process parameters which effect material removal. The RMS relationship was found to be an effective correlation for volumetric flow rate of media and the orifice diameter of various parts, however the specific effects of material removal and flow rate are confounded in the RMS equation states above.
While the RMS of acoustic emissions stated above were found to correlate reasonably well with surface finish and material removal, RMS did not correlate with the complex fluid flow profiles used to determine abrasive flow machining processing completeness, especially in complex passages and geometries of flow. Accordingly, it is an object of the invention to provide method and apparatus to use acoustic emission signals to control abrasive flow machining processes. It is another object of the invention to provide a method which is effective and reliable for controlling abrasive flow machines. It is also an objective of the present invention to provide a consistent method for separating acoustic emission signal into various components to establish media flow rates and material removal.